1. Field of the Invention
The present invention relates to plasma based photon sources. Such sources may be used for example to provide high brightness illumination in methods and for metrology usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Background Art
Photon sources according to the invention may find application in a wide range of situations. As an example application, we will describe use of the invention as a light source in metrology. As a particular field of application of metrology, we shall refer for the sake of example to metrology in the manufacture of devices by lithography. The terms ‘light’ and ‘light source’ may be used conveniently to refer to the generated radiation and the photon source itself, without implying any limitation to radiation of visible wavelengths.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation. From these measured properties a property of interest of the target can be determined.
In one commercially available metrology apparatus, the light source is a xenon (Xe) arc-discharge lamp. Light from this lamp is imaged onto the measurement target through an illumination branch of the apparatus sensor, the last stage of which consists of a high-NA objective. The measurement spot may have a diameter of 25 μm, for example. The spectral distribution of the radiation may be broadband or narrowband in nature, and wavelengths may be in the near infrared, visible and/or ultraviolet bands. The time required for each measurement depends in practice on the brightness of the light source at a given wavelength or wave range. Future generations of apparatus are desired to provide an increased spectral bandwidth and sensor design with lower transmittance, while keeping the measurement time the same or shorter. Significant source brightness improvements are necessary to fulfill these requirements.
Increasing brightness is not achieved simply by increasing the total source power. To increase brightness, a higher power must be delivered into the same small spot size. Etendue is a measure of how ‘spread out’ a rays are in an optical system. A fundamental property of optical systems is that ‘etendue’ never decreases through the system. The optical etendue at the target side of the optical system in the metrology apparatus is very small (due to the small spot size). Therefore the light source must deliver all its energy in a very small etendue, in order to provide a real increase in usable brightness.
Plasma-based photon sources, for example laser driven light sources (LDLS) offer higher brightnesses. Plasmas are generated in a gaseous medium by the application of energy through electric discharge, and laser energy. However, the plasma has a finite physical extent and increasing brightness is still a challenge with these sources.